In existing lighting an HVAC systems, the circuit powering these systems are commonly controlled by wall switches placed in easily accessible locations, such as adjacent to doorways, and the like. Insitutions, businesses, offices, and commercial establishments have found that a great deal of power is consumed to light and ventilate rooms and areas that are unoccupied for relatively long periods. Thus it is preferred for energy conservation reasons, as well as security purposes, that these systems be turned on automatically when a room is entered and that these systems be definitely turned off a short time after the room is completely vacated. To fill this need, occupancy sensors utilizing ultrasonic sensors, ambient noise sensors, infrared sensors, and the like have been developed in the prior art to detect human presence in a room and to switch on and off the relatively large loads comprises of lighting and HVAC circuits.
With regard to converting a typical wall light switch to automatic operation by installation of an occupancy sensor in the wall switch box, it first must be noted that generally only two wires are fed into the wall switch box: the hot leg feed from the utility power supply to the switch, and the wire extending from the switch to the load. Generally, the other side of the load is connected directly to the neutral leg of the AC power supply without returning to the switch box. Thus the switch box is provided with a hot leg to supply the occupancy sensor, but there is no neutral leg to connect to the sensor to complete in the circuit to the sensor. Extending a third wire from AC neutral to the wall box is an extremely costly and time consuming task, due to the fact that wall and/or ceiling surfaces will need to be breached and reclosed, in non-conduit systems, and the wire may need to be pulled through existing conduit.
In this setting, a conventional circuit employing a transformer and switching relay cannot be used, due to the fact that a transformer requires connection between the hot and neutral legs of the AC utility power supply. Indeed, the only common type of switching system that can be used is an electronic switching circuit comprised of a triac and/or diac device. However, the triac and diac devices create electronic noise and also generate radio frequency interference which can be detrimental to the sensitive communications and computer equipment now used in many office and commericial buildings. In addition, these devices are unstable, due to the surge currents and voltages as high as 10,000 volts which occur in electrical systems. These surges can destroy the triac and diac devices. Furthermore, these devices are constantly drawing current and creating heat, whether or not the load is switched on. Many consumers are not favorably disposed towards an electrical system which maintains the wall switch box in a state of perpetual heating that is clearly palpable to the touch.